Add a mising closing curly brace (#1588)

in *Unique IDs* section, the `strings.json` example require one more closing curly brace to be valid json

docs/config_entries_config_flow_handler.md

@@ -79,6 +79,7 @@ Should the config flow then abort, the text resource with the key `already_confi
     "abort": {
       "already_configured": "Device is already configured"
     }
+  }
 }
 ```